All cells are covered with a dense and complex array of sugar chains. Sialic acids (Sias) are a family of nine-carbon sugars that are typically present at the outermost units of these sugar chains. By virtue of their terminal position, sialic acids act as binding sites for many exogenous and endogenous receptors such as the Influenza viruses and the Siglec family of endogenous proteins. Such sugars are thus useful drug targets for the prevention and treatment of infection. They are also involved in various biological and pathological processes such as neuronal plasticity and cancer metastasis. In many of these instances, the precise structures of the sialic acid and the residues it is attached to play critical roles. Thus, studying sialic acid functions is of great biological importance, and antibodies specific for sialic acids are valuable tools in elucidating their biological functions. More specifically, these antibodies would be useful to screen normal human tissues for traces of Neu5Gc, which may be incorporated from certain dietary sources (red meat and diary products), and may also be associated with certain disease states, such as cancer and heart disease.
Sialic acids are typically found as the outermost-units on the mammalian cellular glycocalyx, and on secreted glycoproteins. (Gottschalk, A. (1960), The Chemistry and Biology of Sialic Acids, University Press, Cambridge, U.K.; Rosenberg, A. and Schengrund, C. (1976), Bilogy of Sialic Acids, Plenum Press, New York, N.Y.; Schauer, R. (1982) Adv. Carbohydr. Chem. Biochem. 40:131-234; and Angata, T. and Varki, A. (2002) Chem. Rev. 102:439-470.) The most common Sias are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Cellular Neu5Gc is generated by hydroxylation of the sugar nucleotide donor CMP-Neu-5Ac to CMP-Neu-5Gc, catalyzed by CMP-Neu5Ac hydroxylase (CMAH). (Shaw, L. and Schauer, R. (1988) Biol. Chem. Hoppe-Seyler 369:477-486; Kozutsumi, Y., Kawano, T., Yamakawa, T. and Suzuki, A. (1990) J. Biochem. (Tokyo) 108: 704-706; and Muchmore, E. A., Milewski, M., Varki, A., and Diaz, S. (1989) J. Biol. Chem. 264: 20216-20223.)
Although Neu5Gc is a major Sia in most mammals (including our closest evolutionary relatives, the great apes; Muchmore, E. A., Diaz, S. and Varki, A. (1998) Am. J. Phys. Anthropol. 107: 187-198), it is thought to be absent in healthy humans (Gottschalk, et. al.; Rosenberg, et al.; and Schauer, supra). Indeed, humans generate immune responses against intravenously administered molecules carrying Neu5Gc, e.g. the “serum sickness” reaction to equine anti-thymocyte globulin therapy (Higashi, H., Naiki, M., Matuo, S. and Okouchi, K. (1977), Biochem. Biophys. Res. Comm., 79: 388-395; and Merrick, J. M., Zadarlik, K. and Milgrom, F. (1978), Int. Arch. Allergy Appl. Immunol. 57: 477-480). These findings are explained by a human-specific inactivating mutation in the CMAH gene that occurred 2.5-3 million years ago (Chou, H. H., Takematsu, H., Diaz, S., Iber, J., Nickerson, E., Wright, K. L., Muchmore, E. A., Nelson, D. L., Warren, S. T. and Varki, A. (1998), Proc. Natl. Acad. Sci. USA 95:11751-11756; Irie, A., Koyama, S., Kozutsumi, Y., Kawasaki, T. and Suzuki, A. (1998), J. Biol. Chem. 273:15866-15871; Varki, A. (2002), Yearbook Phys. Anthropol. 44:54-69; and Chou, H. H., Hayakawa, T., Diaz, S., Krings, M., Indriati, E., Leakey, M., Paabo, S., Satta, Y., Takahata, N. and Varki, A. (2002), Proc. Natl. Acad. Sci. USA 99:11736-11741).
Despite no known alternate pathway for Neu5Gc synthesis in humans, antibodies have been used to claim its presence in some human cancers and in human fetal meconium. (Hirabayashi, Y., Kasakura, H., Matsumoto, M., Higashi, H., Kato, S., Kasai, N. and Naiki, M. (1987), Japan J. Cancer Res. 78:251-260; Higashi, H., Hirabayashi, Y., Fukui, Y., Naiki, M., Matsumoto, M., Ueda, S. and Kato, S. (1985), Cancer Res. 45:3796-3802; Miyoshi, I., Higashi, H., Hirabayashi, Y., Kato, S. and Naiki, M. (1986), Mol. Immunol. 23: 631-638; Marquina, G., Waki, H., Fernandez, L. E., Kon, K., Carr, A., Valiente, O., Perez, R. and Ando, S. (1996), Cancer Res. 56: 5165-5171; Devine, P. L., Clark, B. A., Birrell, G. W., Layton, G. T., Ward, B. G., Alewood, P. F. & McKenzie, I. F. C. (1991), Cancer Res. 51:5826-5836; and Kawachi, S., Saida, T., Uhara, H., Uemura, K., Taketomi, T. and Kano, K. (1988), Int. Arch. Allergy Appl. Immunol. 85:381-383.) However, the specificity of the polyclonal antibodies used was not well defined.
One study using mAbs failed to detect Neu5Gc in human tumors and tissues (Furukawa, K., Yamaguchi, H., Oettgen, H. F., Old, L. J. and Lloyd, K. O. (1988), J. Biol. Chem. 263:18507-18512). However, these mAbs were specific for Neu5Gc only in the context of underlying structural motifs. Another mAb thought to be specific for Neu5Gc cross-reacts with some sulfated glycolipids (Vázquez, A. M., Alfonso, M., Lanne, B., Karlsson, K. A., Carr, A., Barroso, O., Fernández, L. E., Rengifo, E., Lanio, M. E., Alvarez, C., Zeuthen, J. and Pérez, R. (1995), Hybridoma 14:551-556). Meanwhile, some reports claim chemical proof for Neu5Gc in human tumors Marquina, et al., Devine, et al., supra, and Kawai T., Kato, A., Higashi, H., Kato, S. and Naiki, M. (1991), Cancer Res. 51:1242-1246). Overall, prior data is inconclusive about the frequency and distribution of Neu5Gc expression in tumors.
Human biosynthetic pathways could theoretically allow exogenous Neu5Gc to be metabolically incorporated (Varki A., et al., supra, and Oetke, C., Hinderlich, S, Brossmer, R., Reutter, W., Pawlita, M. and Keppler, O. T. (2001), Eur. J. Biochem. 268:4553-4561). Indeed, human cells cultured in fetal calf serum express cell surface Neu5Gc in small amounts (Muchmore, et al., and Furukawa, et al., supra). However, it is not known if this represents passive adsorption of serum glycoconjugates or metabolic incorporation. Although earlier studies claimed the absence of Neu5Gc from normal human tissues, a small HPLC peak was noted at the elution time of Neu5Gc in extracts from human organs (Muchmore, et al., supra). More recent studies as discussed elsewhere herein have established the pathway for uptake and incorporation of Neu5Gc by human cells.
Anti-Neu5Gc antibody levels in human sera have also been reported, primarily in patients with various diseases. The presence of such antibodies in normal individuals was considered to be rare. However, assays used in all previous studies lacked absolute specificity for Neu5Gc and also suffered from very high background signals. Thus, they lacked the sensitivity to detect low levels of anti-Neu5Gc antibodies and failed to be certain about the specificity of the reacting antibodies. For example, one study used high molecular weight glycoproteins (HMWG) extracted from bovine erythrocytes as a target without a true negative control, and arbitrarily defined a positive result as an absorbance value of more than 0.5. Thus, only sera with very high reactivity, such as those of patients with certain disease states, would have been identified as positive. Another study that also utilized HMWG as the target, without a true negative control, defined a positive result as the value obtained after subtracting the results obtained using non-coated wells from results obtained using HMWG coated wells. The propensity for serum proteins to bind non-specifically to non-coated wells would have also given a high background in this assay, again making it difficult to identify weakly positive-samples. In both assays the specificity of the reacting antibodies would also not be clearly defined, since cross-reactivity with other bovine molecules could occur.
Accordingly, the present invention relates to detection and analysis of Neu5Gc content of biological materials, such as food and human clinical samples. It also relates to the study of induced and natural antibodies that are specific for Neu5Gc, and their use in the study of Neu5Gc in the laboratory, as well as the detection of Neu5Gc antibodies in clinical specimens. The invention also relates to general methods for selecting anti-sialic acid specific antibodies.